The present invention relates generally to the cleaning of electrodes and more specifically to a method for cleaning electrodes used in electronic flame off methods of bonding ball formation.
Formation of bonding balls on bonding wire is commonly done with the electronic flame off (EFO) technique. This involves moving an electrode near the tip of the bonding wire, and causing an electric arc to pass across the gap between them. This arc melts the tip of the bonding wire, causing a ball to form thereon.
Until very recently, this technique has proved successful only with gold bonding wire. It would be very desirable to use aluminum bonding wire instead of gold because of its lower cost and greater availability. However, the art has only recently advanced to the point where aluminum ball bonding is a feasible industrial process.
Feasible methods for aluminum ball bonding using the electronic flame off technique have been described in U.S. Pat. No. 4,387,283, APPARATUS AND METHOD OF FORMING ALUMINUM BALLS FOR BALL BONDING, issued to Peterson et al., on June 7, 1983, and in a pending U.S. patent application SELF STARTING CURRENT CONTROL DISCHARGE BONDING WIRE MAKER, Ser. No. 248,775, filed Mar. 30, 1981, now abandoned, by James L. Landes, and assigned to the assignee of the present application, both of which references are herein incorporated by reference. These references show a method for forming an aluminum bonding ball by electronic discharge, and surrounding the arc region with argon gas.
A certain amount of aluminum oxide forms on the EFO electrode each time a bonding ball is formed. Due to electrode contamination, the quality of the bonding balls thus formed gradually degrades. Degradation occurs because aluminum oxide is an insulator and interferes with the normal functioning of the electrode tip.
Mechanical scraping of the electrode tip is one possible method of removing the aluminum oxide which has been deposited. However, this method has serious drawbacks in that continual mechanical adjustments would be required to avoid excessive damages to electrode tip. Additionally, the mechanical apparatus needed to scrape the electrode tip would need to be located in the automated ball bonding machinery near the EFO electrode's normal operating position, a location where space is already at a premium.
It would therefore be desirable to provide a method for removing contaminants from the tip of the EFO electrode. It would be desirable that such a method be simple, cheap, and easy to use. It would further be desirable that such a cleaning method cause a minimum amount of damage to the electrode tip, thus extending its life. It would be further desirable that the cleaning method be compatible with the use of automated ball bonding machinery, and be small in order to converse space within the automated ball bonding machinery. The cleaning method should also operate without increasing the cycle time of the bonding machinery.
It is therefore an object of the present invention to provide a method for cleaning an EFO electrode which includes all of the above desirable qualities.
In accordance with the present invention, a cleaning electrode is provided at a position slightly removed from the normal operating position of the EFO electrode. The EFO electrode is periodically removed from its normal operating position to a position adjacent to cleaning electrode. An electric discharge is caused to flow across the gap between the cleaning and EFO electrodes, with the EFO electrode biased negative with respect to the cleaning electrode. The region near the gap between electrodes is filled with a noble gas, preferably argon. The gas is ionized and impacts on the EFO electrode with sufficient energy to remove the thin aluminum oxide layers which have formed thereon.
The novel features which characterize the present invention are defined by the appended claims. The foregoing and other objects and advantages of the present invention will hereinafter appear, and for purposes of illustration, but not of limitation, two preferred embodiments are shown in the drawings.